This invention relates to a method and apparatus for sealing a microporous plastic material and, more particularly, to such an apparatus in which two lapped end portions of the sheet material are dielectrically heat sealed together.
Traditionally, wet cell batteries employ at least one pair of opposite polarity planar electrodes, normally referred to as plates, which are closely spaced in a parallel relationship and immersed in a liquid electrolyte to form an electrochemical couple. A great majority of these type batteries employ a plurality of electrically opposed polarity plates with the plates being normally arranged in an upstanding face-to-face relation, and with the exact number of plates in a particular battery depending upon the capacity and/or voltage desired.
To avoid adjacent opposite polarity plates from coming into direct physical contact and thereby causing arcing and/or short circuiting, it is necessary to electrically insulate the adjacent plates from each other. To achieve this insulation, electrically insulative material, usually in the form of sheets similar in size to the electrode plates, and commonly referred to as separators, have been positioned between the opposite polarity plates, with the insulative material being permeable or semi-permeable to the liquid electrolyte to sustain the ionic conduction required for the battery action.
In older designs, the separators were formed of rubber, wood, or glass while more recent designs have used microporous plastic materials, including, for example, polyethylene, vinyl resin, and the like, which have excellent insulating capabilities yet are relatively low in weight and cost. An example of such a plastic separator is disclosed in U.S. Pat. No. 3,696,061, issued on Oct. 3, 1972, in the name of Selsor et al. and assigned to the same assignee as the present application.
In industrial batteries where heavy duty use requires maximum insulation between the positive and negative plates be provided, it has been found advantageous to envelope either the positive or negative plates with an insulating material, formed and folded to dimensions dictated by the plate size. In a typical envelope configuration, one side of the envelope is formed continuously and the other side is formed by the end portions of the sheet which are overlapped and sealed, so that the envelope has completely insulating sides to prevent lead dendrites from forming from the negative to the positive plates when the battery is charging and discharging in service.
Although battery separators of a plastic material of the general type disclosed above and as specifically disclosed in the aforementioned Selsor et al. patent have proven to be quite satisfactory, the inherent brittleness of this material requires special care during handling. As a result, it has been found difficult to fold the material into the desired envelope configuration and to heat seal the above-mentioned end portions on a high production, relatively inexpensive basis.
In U.S. patent application Ser. No. 535,980 filed on Dec. 23, 1974, now Pat. No. 4,002,417 by the same inventor as the present invention and assigned to the same assignee as the present invention, an apparatus is disclosed which folds a relatively brittle plastic sheet material into an envelope or sleeve-like configuration with one side of the envelope being formed by the overlapping end portions of the sheet. In order to complete the manufacturing process, it thus becomes desirable to seal these overlapping end portions in order to form the completed envelope.